Dye-sensitized photooxidation has been extensively investigated, but has not been suggested as useful for the oxidation of simple olefins to the corresponding epoxides.
P. D. Bartlett and coworkers (J. Am. Chem. Soc., 98:14, 4193 (1976) and Tetrahedron Letters, 33, 2983 (1978)) have investigated the photooxidation of aryl olefins and cycloaliphatic olefins. They report that, while epoxidation occurs with alpha-diketone sensitizers, there is often substantial to complete destruction of the sensitizer. From their studies it is clear that the photooxidation of simple olefins was not believed possible or feasible.
Dye-sensitized photooxidation reactions of olefins have also been investigated by C. S. Foote (Acc. Chem Res, 1, 104 (1968)). Under most conditions, the predominant oxidation products from olefins containing allylic hydrogen atoms are the corresponding hydroperoxides. But as found by Bartlett et al (supra) photooxidation of highly substituted or hindered olefins in the presence of alpha-diketones often affords moderate to high yields of the corresponding epoxides.
T. Sasaki (J. Am. Chem. Soc., 103:13, 3882 (1981)) succeeded in preparing propylene oxide via the photochemical oxidation of propylene with oxygen in acetonitrile in the presence of sulfur dioxide. The reaction appears to proceed via a sulfur dioxide charge transfer complex completely unrelated to the instant invention and gives as a major product poly(propylene sulfonate). Sasaki specifically states that "propylene oxide is not very stable under the reaction conditions and is gradually converted to some polymeric products". He further observes that, "epoxide formation cannot be observed in solvents with ionization potentials lower than ca. 9.5 eV and/or dielectric constants smaller than ca. 10". More specifically "propylene oxide cannot be obtained in solvents such as benzene . . . (and) . . . dichloromethane", among others.
J. P. Shepherd, J. Org. Chem. 48, 337 (1983), describes the photoepoxidation of propylene at elevated pressures when sensitized by alpha-diketones to yield propylene oxide.
Epichlorohydrin is a crucial raw material used in the high volume manufacture of components of epoxy resins. The present commercial methods for the manufacture of epichlorohydrin require the use of relatively strong oxidizing agents such as hydroperoxides, hypohalites, and the like. The utilization of these oxidizers requires costly technology to enable the safe handling and regeneration of the oxidizing agent as well as expensive treatment of the resulting by-products.
Propylene oxide (methyloxirane) is another important commercial intermediate. It is used in the preparation of polyethers and ultimately polyurethanes, as well as in the preparation of propylene glycols. Propylene oxide, too, requires the use of strong oxidizers for commercial manufacture.
A direct oxidation method using air or oxygen as the oxygen source and a direct oxidation agent would have a great impact on the existing technology with concomitant economic benefits.